Explanation of emscripten options (#478)

* Explanation of emscripten options.

* Review fixes.

docs/en/manuals/app-manifest.md

@@ -6,6 +6,7 @@ brief: This manual describes how the app manifest can be used to exclude feature
 # App manifest
 
 The application manifest is used to exclude or control which features to include in the engine. Excluding unused features of the engine is a recommended best practice since it will decrease the final binary size of your game.
+Also application manifest contains some options to control compiling code for HTML5 platform like minimum browser supported version/memory settings that's also can affect on result binary size.
 
 ![](images/app_manifest/create-app-manifest.png)
 
@@ -65,3 +66,35 @@ Select which graphics backend to use.
 * OpenGL - Include only OpenGL.
 * Vulkan - Include only Vulkan.
 * OpenGL and Vulkan - Include both OpenGL and Vulkan. Vulkan will be the default and fall back to OpenGL if Vulkan is not available.
+
+
+## Minimum Safari version (js-web and wasm-web only)
+YAML field name: **minSafariVersion**
+Default value: **90000**
+
+Minimum supported version of Safari. Cannot be less than 90000. For more information look Emscripten compiler options [link](https://emscripten.org/docs/tools_reference/settings_reference.html?highlight=environment#min-safari-version).
+
+## Minimum Firefox version (js-web and wasm-web only)
+YAML field name: **minFirefoxVersion**
+Default value: **34**
+
+Minimum supported version of Firefox. Cannot be less than 34. For more information look Emscripten compiler options [link](https://emscripten.org/docs/tools_reference/settings_reference.html?highlight=environment#min-firefox-version).
+
+## Minimum Chrome version (js-web and wasm-web only)
+YAML field name: **minChromeVersion**
+Default value: **32**
+
+Minimum supported version of Chrome. Cannot be less than 32. For more information look Emscripten compiler options [link](https://emscripten.org/docs/tools_reference/settings_reference.html?highlight=environment#min-chrome-version).
+
+## Initial memory (js-web and wasm-web only)
+YAML field name: **initialMemory**
+Default value: **33554432**
+
+The size of memory that allocated for web application. In case if ALLOW_MEMORY_GROWTH=0 (js-web) - there is a total amount of memory that web application can use. for more information look [link](https://emscripten.org/docs/tools_reference/settings_reference.html?highlight=environment#initial-memory). Value in bytes. Note that value must be a multiple of WebAssembly page size (64KiB).
+That options relates to `html5.heap_size` in game.project [link](https://defold.com/manuals/html5/#heap-size). Option that configured via application manifest is set during compilation and used as default value for INITIAL_MEMORY option. Value from game.project overrides value from application manifest and used in runtime.
+
+## Stack size (js-web and wasm-web only)
+YAML field name: **stackSize**
+Default value: **5242880**
+
+The stack size of application. For more information look [link](https://emscripten.org/docs/tools_reference/settings_reference.html?highlight=environment#stack-size). Value in bytes.